1. Field of the Invention
This disclosure relates generally to ramp analog-to-digital converters, and in particular, but not exclusively, those used in the readout circuitry of a complementary metal-oxide-semiconductor (CMOS) image sensor.
2. Background Information
Image sensors are ubiquitous. They are widely used in digital still cameras, digital video cameras, cellular phones, security cameras, medical devices, automobiles, and other applications.
Many image sensors have an image quality that is limited by a number of factors. One example such factor is temporal noise. For example, temporal noise (including various readout noises and quantization noise) can significantly limit the image quality of an image sensor application. Various methods can be implemented to reduce noise. One example for reducing temporal noise is to enlarge the physical device area on the substrate. However, this is typically not feasible given the layout and size constraints in many image sensors.
Another method for reducing temporal noise in an image sensor is with oversampling analog-to-digital converters (ADCs) with noise shaping, such as with sigma-delta ADCs. In yet another method, the average is taken of multiple samples of a pixel output with a fixed number of samples, either with an integrator (e.g., analog multiple sampling), or after the ADC (e.g., digital multiple sampling). However, conventional multiple sampling typically comes with additional circuit cost and the multiple analog-to-digital conversions inherently becomes slower than ADCs that sample just once.